I put two lowers together it took about thirty minutes each takeing my time.
I think it depends how familiuar your are with the Ar-15/M16 assemblys.
Not hard and they have assemblie instructions on site and others.
Never did an Upper yet but had a friend who did he said took him about an hour with the right tools and tourqe specifications..Good Luck

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Its a very good design for ease of assembly (except for the front pivot pin detent having a tendency to fly across the room). Take your time and have a reference book with you. A little tool you can make that will save you a lot of time is a roll pin driver. Just take a section of 3/8" rod about 4-5" long and drill a hole in one end a little bit bigger than the roll pin you intend to drive. Then smooth out both ends and round them off slightly. drive the pin in most of the way with the hole end, then flip it over and drive it the rest of the way with the solid end. For the bolt hold open latch roll pin, make a pin driver with the hole slightly to one side to give you more clearance. Also you can put electrical tape over any area that might be struck by the hammer or pin driver to protect the guns finish. Most of the kits will have the upper already put together, so just check to make sure the gas key is staked on, then you're good to go.

With a built upper, stripped lower, and all the lower parts.
Tools: Hammer
Small punches
Pliers with electrical tape on the jaws to protect the receiver finish.
Needle nose pliers
Took maybe 20 minutes to figure out where everything went and install it?

Using a 1/4" clevis pin from a hardware store when installing the pivot pin detent & spring will keep you from launching it across the room. It works exactly the same way that the $15 Brownell's tool, but only costs about .25.

Using a 1/4" clevis pin from a hardware store when installing the pivot pin detent & spring will keep you from launching it across the room. It works exactly the same way that the $15 Brownell's tool, but only costs about .25.